1. Field of the Invention
The present invention relates to a method for producing zirconia fused cast refractories which are useful mainly for a floor or a side wall of a glass tank furnace, which is in contact with molten glass.
2. Discussion of Background
Fused cast refractories have a dense structure as compared with bonded refractories (usual fired refractories), and they are excellent in corrosion resistance by virtue of the dense structure. Therefore, fused cast refractories are used mainly at a portion of a furnace where a corrosion action is vigorous. Zirconia fused cast refractories are excellent particularly in a corrosion resistance against molten glass, and they are frequently used for a floor and a side wall of a glass tank furnace which is in contact with molten glass.
Fused cast refractories which are practically used, have a shape of a rectangular parallelpiped in many cases. However, at some portions such as corner portions of a glass tank furnace, a polyhedron having a larger number of faces than the rectangular parallelpiped (hexahedron), may be used. As refractories for a glass tank furnace, it is common to use refractories which have high corrosion resistance against molten glass and which are less likely to generate blisters in the molten glass, at a portion which is in contact with molten glass, so that no defects will be introduced into the glass as far as possible. Further, in a case where the refractories are inhomogeneous, they may be arranged so that the structural portion of the refractories which has high corrosion resistance against molten glass and which is less likely to form blisters in molten glass, is located on the side of molten glass.
As typical zirconia fused cast refractories useful for a glass tank furnace, so-called AZS type zirconia fused cast refractories comprising ZrO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2 and a small amount of an alkali component (for example, of a composition comprising from 33 to 41 wt % of ZrO.sub.2, from 46 to 50 wt % of Al.sub.2 O.sub.3, from 12 to 16 wt % of SiO.sub.2 and from 0.3 to 1.8 wt % of alkali metal oxides) may be mentioned. Recently, demand for high quality glass has increased for e.g. electronic parts, and accordingly, use of high zirconia fused cast refractories containing from 88 to 97 wt % of a ZrO.sub.2 component is increasing, which are excellent in the corrosion resistance against molten glass and which are believed to be less likely to introduce defects in glass by elution of the refractories into molten glass.
The AZS type zirconia fused cast refractories are composed of monoclinic ZrO.sub.2 crystals (mineral name: baddeleyite; although they are monoclinic crystals at room temperature, they undergo transformation to tetragonal crystals at a high temperature), .alpha.-Al.sub.2 O.sub.3 crystals (corundum) and a SiO.sub.2 rich matrix glass containing an alkali component. This matrix glass is present in the refractories as a glass having a proper viscosity in a temperature range of from 900 to 1200.degree. C. within which ZrO.sub.2 crystals undergo reversible crystal transformation from monoclinic crystals to tetragonal crystals accompanied with a volume change, and it functions as a cushion which absorbs and relaxes strains formed in the refractories due to the volume change of the ZrO.sub.2 crystals and thus prevents formation of cracks in the refractories. The function performed by the matrix glass in the refractories, is the same also in the high zirconia fused cast refractories, and the property of the matrix glass for relaxing the strains is more important, since the amount of the matrix glass is relatively small.
Such zirconia fused cast refractories are usually prepared by a method which comprises charging a starting material prepared to have a predetermined chemical composition into an electric arc furnace provided with graphite electrodes, arc-melting the starting material, pouring the meltage into a mold made of e.g. graphite, having a predetermined internal size and having previously embedded in a thermal insulator, followed by cooling to solidify.
With zirconia fused cast refractory, when the meltage solidifies upon cooling, its volume decreases from 20 to 30%. Accordingly, an inlet port having a certain capacity, which is a so-called riser, is provided to supply the meltage to the portion shrinked upon solidification of the meltage. However, the supply of the meltage to the portion where the meltage solidified, will necessarily terminate, whereupon voids will be formed in the solidified refractory, whereby voids will be formed in the refractory. Such voids are usually present in a cluster at the upper center near the inlet port, where the meltage was introduced, of the fused cast refractories.
When zirconia fused cast refractories are to be used for a glass tank furnace, the presence of such void clusters impairs the useful life of the refractories and will be a cause for formation of defects in the glass. Accordingly, void free refractories (hereinafter referred to as VF refractories) obtained by cutting off the portion including void clusters (which usually extend to cover about 50 vol % of the cast refractories), are used for a glass tank furnace in many cases.
Various measures have been taken to improve the yield of the products and to produce high quality glass having no substantial defects such as blisters or stones. However, even if VF refractories of zirconia fused cast refractories containing from 88 to 97 wt % of a ZrO.sub.2 component, which are regarded to be the best at present, are used after scraping off the surface formed during casting, it is still difficult to completely eliminate defects formed in the glass.
The present inventors have carefully examined the state where blisters generate in molten glass at the contact between the molten glass and the high zirconia fused cast refractories containing from 88 to 97 wt % of a ZrO.sub.2 component. As a result, it has been found that in the dense structure of the high zirconia fused cast refractories, a slightly dark gray layer structure visually observed on a cross-sectional polished surface, having a thickness of at most 1 mm (hereinafter referred to as worm tracing or WT), is present, in which the matrix glass and fine voids (mostly not larger than 1 mm) are accumulated in a layer form, and a number of such WT are present along the surface of the refractories, which was in contact with a side surface of the mold during solidification of the high zirconia fused cast refractories. Further, such WT is likely to be a starting point of abnormal corrosion of the refractory, and it is also likely to be a point for generating blisters in molten glass.
Further, as a result of careful examination of the cross-sectional polished surface of the high zirconia fused cast refractories containing from 88 to 97 wt % of a ZrO.sub.2 component, it has been found that no such WT is observed at the portion of the refractories located in a distance of from 6 to 8 cm from the lower surface of the refractories which was in contact with the bottom of the mold during solidification of the refractories.